The present invention relates to a paste solder which comprises a powdered solder uniformly admixed with a flux. More particularly, it relates to a paste solder which leaves a minimized amount of flux residue after soldering and which is suitable for use in an inert or reducing atmosphere.
Paste solder (also called cream solder) is a paste-like, uniform mixture of a powdered solder and a pasty flux. It has rheological properties suitable for printing and adhesion. Therefore, it can be readily and accurately applied by printing, and the applied solder can hold the elements mounted thereon before soldering due to its adhesiveness without the use of an adhesive. A paste solder is also advantageous in that it can be used to solder many elements simultaneously by employing an atmosphere-heating means such as a reflow furnace. Due to these characteristics, paste solder has contributed to advance in surface mounting techniques for the manufacture of printed circuit boards.
Soldering with a paste solder is performed, for example, by applying the paste solder to the areas to be soldered on the surface of a circuit board by printing using a stencil and a squeegee. One or more electronic elements to be mounted on the surface of the circuit board, such as IC modules, chip capacitors, chip resistors, or the like are then placed on the printed paste solder and temporarily secured in place by the adhesive power of the solder. Subsequently, the solder is heated to melt in a heating apparatus such as a reflow furnace, thereby soldering the electronic elements to the circuit board to form an electronic device.
Thus, surface mounting using a paste solder makes it possible to solder electronic elements with a decreased number of steps and an increased accuracy. Therefore, it is growing increasingly popular and becoming the prevalent soldering method as the demand for high-density mounting and cost savings in the manufacture of electronic devices increases.
A paste solder which has conventionally been used consists of about 85%-92% of a powdered solder and about 8%-15% of a flux on a weight basis. The flux comprises the following ingredients:
50%-70% by weight of a rosin or its derivative,
2%-7% by weight of a thixotropic agent,
0.1%-5% by weight of an activating agent, and
30%-45% by weight of a solvent.
The rosin or rosin derivative is the main fluxing agent of a flux and it constitutes carrier components of a flux along with the remaining ingredients other than the solvent.
The paste solder is prepared by heating the ingredients of a flux together to dissolve the carrier components in the solvent and form a homogeneous solution. The solution is then cooled to form a paste and the resulting pasty flux is uniformly mixed with a powdered solder to give a paste solder.
Such a paste solder should meet the following requirements.
(1) No separation between the powdered solder and flux.
(2) Good printability, i.e., capable of smooth printing by means of a stencil and squeegee.
(3) Good adhesiveness, i.e., capable of holding and temporarily securing electronic elements when they are mounted on the printed paste solder.
(4) Good retention of the printed solder shape before or even after it is preliminary heated.
(5) No solder balls formed when the printed solder having electronic elements temporarily secured thereon is heated by a reflow furnace, for example.
(6) A flux residue remaining after soldering with the paste solder is non-corrosive and has good electrical insulating properties.
(7) The flux residue can be readily dissolved out and removed by washing with a washing solvent, which is usually a halogenated hydrocarbon solvent.
A flux residue remaining after soldering has the following undesirable effects.
(a) When a relatively large amount of a flux residue remains after soldering, it not only worsens the appearance of soldered portions but also makes it impossible to perform a continuity test with a checking pin to confirm that the mounted elements are well soldered so as to operate the device satisfactorily.
(b) Some fluxes are corrosive or hygroscopic in nature due to their formulations. When an electrical current is passed through a circuit soldered with such a flux, the remaining flux residue of such a flux may cause corrosion or migration of the solder or decrease the insulation resistance between conductors. This is particularly undesirable in an industrial or life support system which must operate reliably for many years.
In order to eliminate these undesirable effects of a flux residue, it is common, after soldering with a conventional paste solder, to wash the soldered electronic device with an organic halogenated solvent including a fluorinated and/or chlorinated hydrocarbon to remove the flux residue. However, the use of such a halogenated solvent is now regulated to prevent an adverse environmental impact, and it is expected to be completely prohibited in the future.
It is conceivable to use water or another organic solvent in place of an organic halogenated solvent in the washing step after soldering. However, if water is used, it is necessary to dispose of the waste water by a complicated process, while other organic solvents involve problems with respect to flammability or costs. Therefore, replacement of a halogenated solvent by water or other organic solvent has scarcely been realized.
As described above, a flux used in a conventional paste solder contains a rosin or its derivative as a main fluxing agent in a relatively large amount (on the order of 50% by weight or more). Correspondingly, the amount of the solvent is as low as 45% by weight at most. See, Japanese Patent Publication (JPB) No. 61-15798(1986) and Japanese Patent Laid-Open Applications (JPA) Nos. 56-154297(1981), 57-118891(1982), 59-153594(1984), 60-180690(1985), 60-257988(1985), 61-78589(1986), and 61-108491(1986).
Accordingly, a water-soluble organic solvent having a relatively low viscosity is commonly used to prepare a flux for a paste solder in order to provide the resulting paste solder with desirable properties such as printability, adhesion for holding electronic elements, and retention of printed shape. Examples of such a solvent used in the prior art are diethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, triethylene glycol monomethyl ether, tripropylene glycol monomethyl ether, ethylene glycol monophenyl ether, propylene glycol monophenyl ether, diethylene glycol monohexyl ether, diethylene glycol mono-2-ethylhexyl ether, ethylene glycol, propylene glycol, dipropylene glycol, butanediol, hexylene glycol, phenol, benzyl alcohol, and the like.
Due to the high rosin content of the flux, the conventional paste solder inevitably leaves a considerable amount of a flux residue after soldering and therefore it has been essential to conduct the above-mentioned washing procedure to remove the residual flux.
JPA 2-25291(1990) discloses a paste solder for use in a reducing atmosphere which comprises a powdered solder and at least one binder material including an alcohol. Since it is a flux-free solder, it is necessary to conduct soldering therewith in a reducing atmosphere which functions as a flux. If the paste solder is used in an inert atmosphere, soldering will not be performed satisfactorily due to the absence of a flux. In addition, the solder is a simple mixture of a powdered solder and an organic solvent. Therefore, the pasty nature is soon lost, since the powdered solder settles out in the organic solvent so as to separate therefrom. Naturally, the solder cannot be applied by printing, and even though the solder can be applied to those areas of a board to be soldered, the applied solder does not have adhesion sufficient to hold electronic elements in place on the board.